The production of fiber reinforced composite components, and in particular those components formed of a fiber/resin combination, have traditionally been accomplished by a number of open and closed molding lamination processes, or variations of each. Examples of these components include those used in the boating industry; such as fiber reinforced plastic sheets and parts with a compound shape used to manufacture a hull for a watercraft. These molding processes all involve a fiber reinforcement (e.g., fiberglass pieces) being laid up against a mold (e.g., a female mold) that provides the desired shape for the component, and the impregnation of the fiber with resin or a similar material. After curing, the resin/fiber combination forms a finished part that can be removed from the mold. Apart from these similarities, however, molding processes are distinct in the efficiencies provided by each, as well as in the disadvantages or tradeoffs encountered when choosing a molding process for fabricating a specific type or run of a component.
Open molding lamination processes are traditionally more labor intensive than other molding processes and usually produce less consistent part quality in high volume production. Open molding processes are also falling out of favor with manufacturers due to the high emissions of volatile organic compounds (VOC) realized in part production. One proposed solution was to use component materials that resulted in fewer VOC emissions, but this has resulted in parts having poor physical characteristics. Still, despite these disadvantages, open molding processes—along with vacuum bag molding—are generally the only widely used molding processes that are capable of producing low volume or low run parts in a cost effective manner.
Vacuum bag molding is a type of closed molding technique that involves forming a thin flexible bag to cover the mold upon which the fiber lay up is positioned. The edges of the bag are then clamped, taped or otherwise secured to the mold to form a sealed envelope surrounding the fiber lay up. One or more vacuum supply lines are usually installed within the bag to apply a vacuum on the bag interior concomitant with catalyzed liquid plastic or resin being introduced into the bag through a resin supply line to impregnate the fiber lay up. The vacuum draws the bag against the resin/fiber combination and surface of the mold to shape the combination into the desired part. The resin supply lines are typically positioned to introduce resin either at the perimeter of the part such that the vacuum supply line draws the resin across and through the fiber lay up towards the center of the part, or vice versa, with the resin introduced at the center of the part and vacuum drawing the resin towards the perimeter of the part. Vacuum bag molding can usually be categorized as either utilizing, (1) a thin disposable bag made from sheet film, or (2) a reusable bag made from silicone, both of which are flexible bags. Because the resin and fiber are essentially sealed off from the surrounding environment, vacuum bag molding techniques expose tool operators to significantly fewer VOC's than with open molding processes, which is a significant reason why vacuum bag techniques have gained interest in recent years.
When using a disposable vacuum bag, a peel ply release film and a resin flow/bleeder media must often be stacked atop the fiber lay up below the bag because of the nature of the thin sheet film to conform very tightly to the fiber layer up and make resin flow very difficult. The resin flow/bleeder media facilitates flow of the resin across and through the fiber lay up in a timely manner by essentially forming a resin passageway, and the peel ply film ensures that both the media and peel ply layer itself may be easily pulled off of the finished part without undue effort. Additionally, resin and vacuum distribution lines extending from the supply lines and routed beneath the vacuum bag across the mold are often needed in addition to the resin flow/bleeder media to properly distribute the resin and apply the vacuum draw beneath the tightly drawn thin sheet film. Also, adhesive sealant tape is typically applied around the perimeter of the bag to form an airtight seal with the mold and facilitate proper vacuum operation.
Despite the high quality of the part produced using disposable vacuum bag molding techniques (i.e., having a high fiber to resin ratio), certain disadvantages are apparent. For example, many of the aforementioned components used in disposable vacuum bag techniques—including the vacuum bag having resin and vacuum supply lines integrally formed therewith, the resin flow/bleeder media, the peel ply film, the resin and vacuum distribution lines and the adhesive sealant tape—are disposed of after molding only a single part, making this technique prohibitively expensive for all but high margin parts manufacturing. Significant labor is also necessary when using a disposable bag, as the bag must be made by hand to fit the particular base mold and also installed by hand with the resin flow/bleeder media, peel ply film, resin and vacuum distribution lines and sealant tape at the proper positions for the vacuum draw and resin impregnation of the fiber lay up to work. Furthermore, if the female mold has a complex shape, many pieces of sheet film may need to be cut and bonded together with sealant tape to produce a bag with the desired shape, thereby significantly increasing manufacturing time per part as compared to open molding processes.
Silicone bags implemented in vacuum bag molding techniques provide the distinct advantage of being reusable, such that multiple parts may be produced with the mold using just one bag. This provides a saving in labor as compared to disposable sheet film bags. The silicone bag is typically fabricated from silicone sheet or a brushable silicone liquid, but because silicone alone usually does not have enough strength to withstand the vacuum draw in the molding process, a reinforcement such as nylon mesh or other materials is often laminated with the silicon. Another problem with silicone is that even with a reinforcement, the bag tends to have a relatively short life and is useful for only small part production runs; the silicone bag is easily damaged in the production environment, difficult to have resin and vacuum supply and distribution lines integrated therewith, and tends to expand dimensionally over a number of production runs, requiring significant maintenance to keep the bag usable for the molding process. As with disposable vacuum bags, adhesive sealant tape or other adhesives must still be used when sealing the silicone bag perimeter to the mold to form an airtight seal for vacuum operation. Silicone materials are also expensive, leading to a high initial cost of fabricating the vacuum bag. Also, silicone bags require oven curing in the fabrication of the more durable materials, which adds further to the cost of the process with the additional time, energy and equipment requirements.
Yet another closed molding process, resin transfer molding, involves using rigid male and female molds together to produce fiber reinforced composite parts. A fiber lay up is placed on the female mold and the male mold is brought into contact with the female mold and clamped or otherwise secured therewith so that a closed space is formed between the molds. Then, a mixed resin and catalyst are injected into the closed space under relatively low pressure. Upon curing of the resin, the molds are separated and the part is removed. The resin transfer molding process is more environmentally friendly than traditional open molding processes, with the capture of any VOC's present in the closed space occurring before the molds are separated to reveal the finished part. One significant disadvantage of resin transfer molding, however, is that because the male and female molds are rigid, if the fiber load of the lay up is not precisely the correct quantity at the correct position, structural weakness in the part occur. For example, “dry spots” occur where the resin cannot flow to during the injection process if the fiber density is too high, and if the fiber density is too low, a spot filled with resin will develop. Both dry spots and resin filled spots in finished parts are susceptible to fracture or other structural failures at relatively low force loads. These structural weaknesses are even more important when fabricating large parts, such as boat hull components, where the weight of the part itself may facilitate structural failures. Matched, rigid tooling is very expensive to produce and, therefore, the process is less amenable to changes that may be required for structural, process, or styling updates.
Current closed molding lamination techniques do not provide an economical and reliable solution for fabricating fiber reinforced composite parts, especially with respect to small to medium part runs. A molding technique is needed that balances tool cost considerations with reliability and speed in the part fabrication process.